Active phase separation in mixtures of chemically-interacting particles

Ramin Golestanian
Max Planck Institute for Dynamics and Self-Organisation Göttingen, Germany

Abstract:
Chemical interactions are paradigmatic of both living and synthetic active matter. Biological cells, bacteria, and other microorganisms are known to communicate with each other through production or consumption of signalling molecules that induce chemotaxis. The same behaviour is found for biological enzymes at the nanoscale, and can be mimicked in model systems consisting of catalytic colloids. Importantly, such chemical interactions require activity far from thermodynamic equilibrium, which in turn may manifest itself as a broken action-reaction symmetry in the interactions. Through particle-based simulations as well as a continuum theory, we show that mixtures of chemically-interacting particles undergo a great variety of phase separation phenomena that would be impossible in an equilibrium system. The continuum theory allows us to obtain simple conditions which determine whether or not phase separation will take place, and to characterise the phase-separated states. Our results can be directly related to self-organization in bacterial colonies and cellular tissues; to metabolon formation by enzymes and thus to the understanding and design of biological and synthetic catalytic pathways; and to the engineering of new active materials using catalytic colloids. Besides these direct applications, our results are also a step forward in the fundamental study of phase separation far from thermodynamic equilibrium.